The luminance of an Organic electroluminescent display OLED is directly proportional to a driving current flowing therethrough, and therefore a pixel circuit must provide the OLED with a sustained and stable driving current during a whole frame period. A driving manner of the existing OLED pixel circuit may be divided into a current driving mode and a voltage driving mode, as illustrated in FIG. 1 and FIG. 2, respectively.
In a voltage-mode driving circuit, a current Ioled flowing through a light-emitting device is:
      I    OLED    =            1      2        ⁢                  μ        n            ·      Cox      ·              W        L            ·                                    (                          Vdata              -              Voled              -              Vth                        )                    2                .            
Wherein μn is a mobility of carriers, COX is a capacitance in an oxide layer at a gate, W/L is a width-length ratio of the transistor, Vdata is a data voltage, Voled is an operational voltage of the OLED and is shared by all of sub-pixel units, Vth is a threshold voltage of the transistor, which is a positive value for an enhanced Thin Film Transistor TFT and is a negative value for a depletion TFT. It can be seen from the above equation that the currents would be different if the threshold voltages Vth are different among the different pixel units. If the Vth in a pixel drifts as time elapses, the current would also vary with time and an image sticking may occur. Also, the variations in the current may also be resulted from the differences in the operational voltages of the OLEDs due to the non-uniformity in the OLED devices.
As compared with the voltage-mode driving mode, the current-mode driving mode has advantages as follows. The current Ioled=Idata, and if the threshold voltage of the pixel drifts as the time elapses, the current-mode driving circuit has a capability of self-adjusting the current level of the current so that the current is independent of the Vth of the TFT device, and thus a display which is uniform in space and stable in time can be achieved. The current-mode driving circuit is generally applied to a panel with a small size, however, because of its long driving time. FIG. 3 is an exemplary view illustrating a structure of a current-mode driving circuit for a pixel driving circuit, and FIG. 4 is a timing diagram of the circuit structure shown in FIG. 3. It can be seen from these two figures that the operation of the circuit is divided into two phases of a percharging phase t1 and a light-emitting phase s2. During the t1 phase, an ARVDD is at a low level, a SCAN is at a high level, a transistor M4 is turned off, and a Cs is charged; and during the t2 phase, the ARVDD is at the high level, the SCAN is at the low level, transistors M1 and M2 are turned off, and an OLED emits light. Such a current-mode driving pixel circuit has a great defect in that a charging time of its capacitor is too long so that the display is affected, which may in turn restrain a large scale application of the current-mode driving circuit.